In U.S. Pat. No. 3,900,554, Lyon discloses a non-catalytic system for reducing nitrogen monoxide (NO) in a combustion effluent. Lyon discloses that ammonia and specified ammonia precursors or their aqueous solutions, can be injected into the effluent for mixing with the nitrogen monoxide at a temperature within the range of 1600.degree. F. to 2000.degree. F. In one embodiment of the disclosed process, a reducing agent can be mixed with the effluent to permit the reduction reaction to occur at temperatures as low as 1300.degree. F., thereby assuring avoidance of high temperature oxidation of ammonia to nitrogen monoxide. Lyon discloses that hydrogen is preferred as compared to aromatic, parafinic and olefinic hydrocarbons and oxygenated hydrocarbons, and discloses nothing with regard to the control of ammonia in the final effluent.
In U.S. Pat. No. 3,961,018, Williamson discloses the purification of acid gas-containing streams at low temperatures approaching ambient. Williamson discloses contacting the gas stream with an amine vapor in sufficient concentration such that its partial pressure is at least 5% of the total pressure of the gas stream. This system thus requires large amounts of the treating gas and does not address the issue of ammonia gas in the final effluent.
In a somewhat different environment, Goldstein et al, in U.S. Pat. No. 4,061,597 indicate that temperatures within the range of 1000.degree. F. to 1300.degree. F. are effective when using urea for reducing brown fumes caused by nitrogen dioxide (NO.sub.2) from catalyst treatment effluents. One example in the patent employs a 30 weight percent aqueous solution of urea. There is no disclosure here again of control of ammonia in the final effluent.
In U.S. Pat. No. 4,325,924, Arand et al disclose the non-catalytic urea reduction of nitrogen oxides in fuel-rich combustion effluents. They indicate that under fuel-rich conditions, aqueous solutions of urea at concentrations of greater than 10%, and preferably greater than 20%, are effective nitrogen oxide reducers at temperatures in excess of 1900.degree. F. This is the effluent from staged combustion which results in the production of high levels of carbonaceous pollutants.
In U.S. Pat. No. 4,208,386, on the other hand, Arand et al disclose that for oxygen-rich effluents, the temperature is in the range of from 1300.degree. F. to 2000.degree. F. for urea added dry or as a solution in water alone or with suitable solvent, such as a 1 to 3 carbon alkanol. The alkanoic solvent is said to be a reducing agent which, like hydrogen, carbon monoxide, etc., is said to enable the effective operating temperature to be lowered to below 1600.degree. F. No function, other than carrier for the urea, was disclosed for the water. And, as with the other patents, no mention is made of reducing ammonia levels in the effluent or of any problem they may cause when employing a fuel with a significant sulfur content.
Accordingly, there is a present need for a process which enables the reduction of nitrogen-based pollutants by operating under efficient oxygen-rich conditions which minimize carbon-based pollutants, and yet permits the control of ammonia levels in the final effluent.